[PATCH] mm: clean up soft_offline_page()

[Naoya Horiguchi]

Hi,

I wrote this patch inspired by the discussion about fixing mce_bad_pages bug.
https://lkml.org/lkml/2012/12/7/66
As mentioned by Andrew, this bug seemed to be undetected because of the
messiness of soft_offline_page(), so with this patch we can deal with the problem.

Thanks,
Naoya
---
From: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Date: Thu, 13 Dec 2012 16:08:54 -0500
Subject: [PATCH] mm: clean up soft_offline_page()

Currently soft_offline_page() is hard to maintain because it has many
return points and goto statements. All of this mess come from get_any_page().
This function should only get page refcount as the name implies, but it does
some page isolating actions like SetPageHWPoison() and dequeuing hugepage.
This patch corrects it and introduces some internal subroutines to make
soft offlining code more readable and maintainable.

Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
---
 mm/memory-failure.c | 189 ++++++++++++++++++++++++++++------------------------
 1 file changed, 101 insertions(+), 88 deletions(-)

diff --git v3.7.orig/mm/memory-failure.c v3.7/mm/memory-failure.c
index 8b20278..8cef032 100644
--- v3.7.orig/mm/memory-failure.c
+++ v3.7/mm/memory-failure.c
@@ -1368,7 +1368,7 @@ static struct page *new_page(struct page *p, unsigned long private, int **x)
  * that is not free, and 1 for any other page type.
  * For 1 the page is returned with increased page count, otherwise not.
  */
-static int get_any_page(struct page *p, unsigned long pfn, int flags)
+static int __get_any_page(struct page *p, unsigned long pfn, int flags)
 {
 	int ret;
 
@@ -1393,11 +1393,9 @@ static int get_any_page(struct page *p, unsigned long pfn, int flags)
 	if (!get_page_unless_zero(compound_head(p))) {
 		if (PageHuge(p)) {
 			pr_info("%s: %#lx free huge page\n", __func__, pfn);
-			ret = dequeue_hwpoisoned_huge_page(compound_head(p));
+			ret = 0;
 		} else if (is_free_buddy_page(p)) {
 			pr_info("%s: %#lx free buddy page\n", __func__, pfn);
-			/* Set hwpoison bit while page is still isolated */
-			SetPageHWPoison(p);
 			ret = 0;
 		} else {
 			pr_info("%s: %#lx: unknown zero refcount page type %lx\n",
@@ -1413,23 +1411,45 @@ static int get_any_page(struct page *p, unsigned long pfn, int flags)
 	return ret;
 }
 
+static int get_any_page(struct page *page, unsigned long pfn, int flags)
+{
+	int ret = __get_any_page(page, pfn, flags);
+
+	if (ret == 1 && !PageHuge(page) && !PageLRU(page)) {
+		/*
+		 * Try to free it.
+		 */
+		put_page(page);
+		shake_page(page, 1);
+
+		/*
+		 * Did it turn free?
+		 */
+		ret = __get_any_page(page, pfn, 0);
+		if (!PageLRU(page)) {
+			pr_info("soft_offline: %#lx: unknown non LRU page type %lx\n",
+				pfn, page->flags);
+			return -EIO;
+		}
+	}
+	return ret;
+}
+
 static int soft_offline_huge_page(struct page *page, int flags)
 {
 	int ret;
 	unsigned long pfn = page_to_pfn(page);
 	struct page *hpage = compound_head(page);
 
-	ret = get_any_page(page, pfn, flags);
-	if (ret < 0)
-		return ret;
-	if (ret == 0)
-		goto done;
-
+	/* Synchronized using the page lock with memory_failure() */
+	lock_page(hpage);
 	if (PageHWPoison(hpage)) {
+		unlock_page(hpage);
 		put_page(hpage);
 		pr_info("soft offline: %#lx hugepage already poisoned\n", pfn);
 		return -EBUSY;
 	}
+	unlock_page(hpage);
 
 	/* Keep page count to indicate a given hugepage is isolated. */
 	ret = migrate_huge_page(hpage, new_page, MPOL_MF_MOVE_ALL, false,
@@ -1439,85 +1459,19 @@ static int soft_offline_huge_page(struct page *page, int flags)
 		pr_info("soft offline: %#lx: migration failed %d, type %lx\n",
 			pfn, ret, page->flags);
 		return ret;
+	} else {
+		set_page_hwpoison_huge_page(hpage);
+		dequeue_hwpoisoned_huge_page(hpage);
+		atomic_long_add(1<<compound_trans_order(hpage), &mce_bad_pages);
 	}
-done:
-	if (!PageHWPoison(hpage))
-		atomic_long_add(1 << compound_trans_order(hpage),
-				&mce_bad_pages);
-	set_page_hwpoison_huge_page(hpage);
-	dequeue_hwpoisoned_huge_page(hpage);
 	/* keep elevated page count for bad page */
 	return ret;
 }
 
-/**
- * soft_offline_page - Soft offline a page.
- * @page: page to offline
- * @flags: flags. Same as memory_failure().
- *
- * Returns 0 on success, otherwise negated errno.
- *
- * Soft offline a page, by migration or invalidation,
- * without killing anything. This is for the case when
- * a page is not corrupted yet (so it's still valid to access),
- * but has had a number of corrected errors and is better taken
- * out.
- *
- * The actual policy on when to do that is maintained by
- * user space.
- *
- * This should never impact any application or cause data loss,
- * however it might take some time.
- *
- * This is not a 100% solution for all memory, but tries to be
- * ``good enough'' for the majority of memory.
- */
-int soft_offline_page(struct page *page, int flags)
+int __soft_offline_page(struct page *page, int flags)
 {
 	int ret;
 	unsigned long pfn = page_to_pfn(page);
-	struct page *hpage = compound_trans_head(page);
-
-	if (PageHuge(page))
-		return soft_offline_huge_page(page, flags);
-	if (PageTransHuge(hpage)) {
-		if (PageAnon(hpage) && unlikely(split_huge_page(hpage))) {
-			pr_info("soft offline: %#lx: failed to split THP\n",
-				pfn);
-			return -EBUSY;
-		}
-	}
-
-	ret = get_any_page(page, pfn, flags);
-	if (ret < 0)
-		return ret;
-	if (ret == 0)
-		goto done;
-
-	/*
-	 * Page cache page we can handle?
-	 */
-	if (!PageLRU(page)) {
-		/*
-		 * Try to free it.
-		 */
-		put_page(page);
-		shake_page(page, 1);
-
-		/*
-		 * Did it turn free?
-		 */
-		ret = get_any_page(page, pfn, 0);
-		if (ret < 0)
-			return ret;
-		if (ret == 0)
-			goto done;
-	}
-	if (!PageLRU(page)) {
-		pr_info("soft_offline: %#lx: unknown non LRU page type %lx\n",
-			pfn, page->flags);
-		return -EIO;
-	}
 
 	lock_page(page);
 	wait_on_page_writeback(page);
@@ -1544,9 +1498,10 @@ int soft_offline_page(struct page *page, int flags)
 	 */
 	if (ret == 1) {
 		put_page(page);
-		ret = 0;
 		pr_info("soft_offline: %#lx: invalidated\n", pfn);
-		goto done;
+		SetPageHWPoison(page);
+		atomic_long_inc(&mce_bad_pages);
+		return 0;
 	}
 
 	/*
@@ -1573,17 +1528,75 @@ int soft_offline_page(struct page *page, int flags)
 				pfn, ret, page->flags);
 			if (ret > 0)
 				ret = -EIO;
+		} else {
+			SetPageHWPoison(page);
+			atomic_long_inc(&mce_bad_pages);
 		}
 	} else {
 		pr_info("soft offline: %#lx: isolation failed: %d, page count %d, type %lx\n",
 			pfn, ret, page_count(page), page->flags);
 	}
-	if (ret)
-		return ret;
+	return ret;
+}
+
+/**
+ * soft_offline_page - Soft offline a page.
+ * @page: page to offline
+ * @flags: flags. Same as memory_failure().
+ *
+ * Returns 0 on success, otherwise negated errno.
+ *
+ * Soft offline a page, by migration or invalidation,
+ * without killing anything. This is for the case when
+ * a page is not corrupted yet (so it's still valid to access),
+ * but has had a number of corrected errors and is better taken
+ * out.
+ *
+ * The actual policy on when to do that is maintained by
+ * user space.
+ *
+ * This should never impact any application or cause data loss,
+ * however it might take some time.
+ *
+ * This is not a 100% solution for all memory, but tries to be
+ * ``good enough'' for the majority of memory.
+ */
+int soft_offline_page(struct page *page, int flags)
+{
+	int ret;
+	unsigned long pfn = page_to_pfn(page);
+	struct page *hpage = compound_trans_head(page);
 
-done:
-	atomic_long_add(1, &mce_bad_pages);
-	SetPageHWPoison(page);
+	if (PageHWPoison(page)) {
+		pr_info("soft offline: %#lx page already poisoned\n", pfn);
+		return -EBUSY;
+	}
+	if (!PageHuge(page) && PageTransHuge(hpage)) {
+		if (PageAnon(hpage) && unlikely(split_huge_page(hpage))) {
+			pr_info("soft offline: %#lx: failed to split THP\n",
+				pfn);
+			return -EBUSY;
+		}
+	}
+	ret = get_any_page(page, pfn, flags);
+	if (ret < 0)
+		return ret;
+	if (ret) { /* for in-use pages */
+		if (PageHuge(page))
+			soft_offline_huge_page(page, flags);
+		else
+			__soft_offline_page(page, flags);
+	} else { /* for free pages */
+		if (PageHuge(page)) {
+			set_page_hwpoison_huge_page(hpage);
+			dequeue_hwpoisoned_huge_page(hpage);
+			atomic_long_add(1 << compound_trans_order(hpage),
+					&mce_bad_pages);
+		} else {
+			SetPageHWPoison(page);
+			atomic_long_inc(&mce_bad_pages);
+		}
+	}
 	/* keep elevated page count for bad page */
 	return ret;
 }
-- 
1.7.11.7

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